The present invention relates to a beam splitting element that divides an incident beam into a plurality of diffracted beams.
Examples of conventional beam splitting elements are disclosed in Japanese patent provisional publications Nos. HEI 5-323110 and HEI 7-225305. The conventional beam splitting element as disclosed in the publications is provided with diffractive grating having a plurality of elongated grooves and raised portions (ridges) formed in parallel and alternately on a glass substrate. Each of the grooves and the raised portions has a rectangular sectional shape in a plane that is perpendicular to the direction in which the grooves are elongated. Heights and depths of the raised portions and the grooves are constant, while the pitches of the grooves and/or the raised portions are uneven.
However, the conventional beam splitting element as a relatively low diffraction efficiency, which loses energy of the incident light beam. The diffraction efficiencies of the embodiments are in a range of 70%-85% in the publication 5-323110, and 81% in the publication 7-225305.